1. Field of the Invention
The present invention relates to a fixing device used in an image forming apparatus, particularly to the fixing device in which presence or absence of a recording sheet winding around a rotating body is detected in restarting after a jam and the restart can safely be performed.
2. Description of Related Art
For example, Japanese Patent Laid-Open No. 2003-255753 discloses a technique, wherein a noncontact temperature detection unit is provided on the downstream side of a nip portion of a fixing roller, a temperature of a heating roller is controlled by the noncontact temperature detection unit, and it is judged that the jam is generated when the noncontact temperature detection unit detects the temperature lower than the temperature at which the recording sheet passes normally through the nip portion. Japanese Patent Laid-Open No. 2003-255753 also discloses a technique, wherein it is judged that sheet discharge delay jam is generated when the noncontact temperature detection unit detects that the temperature is not lowered even after a predetermined time elapses since the recording sheet enters the nip portion.
According to Japanese Patent Laid-Open No. 2003-255753, the jam can be detected using the noncontact temperature detection unit. Recently, speed-up is increasingly demanded in the image forming apparatus, the recording sheet is inevitably conveyed at high speed and a gap between the recording sheets becomes narrower. For example, the image forming apparatus having performance not lower than 80 sheets per minute or 100 sheets per minute is already developed. In such pieces of high-speed image forming apparatus, when the jam is generated, not only one recording sheet is jammed, but also several recording sheets subsequent to the jammed recording sheet remain in the way of a conveyance path, and sometimes the recording sheet winds around a rotating roller. Accordingly, it is necessary that the jammed recording sheet be removed while all the subsequent recording sheets located in the way of the conveyance path are removed.
In the conventional image forming apparatus, sensors are placed at several points in the way of the conveyance path. After one sensor detects the passage of the recording sheet, when the next sensor detects the passage of the recording sheet within a predetermined time, it is judged that the passage of the recording sheet is normally performed. It is judged that the jam is generated unless the next sensor detects the passage of the recording sheet within the predetermined time. Therefore, the number of jammed recording sheets is not detected and the position of the recording sheet is not correctly detected. Only it is judged that the jammed recording sheet is located between one sensor and the next sensor.
Accordingly, unless a user removes all the jammed recording sheets, sometimes the jam is generated again in restarting the image forming apparatus. Particularly, when the jammed recording sheet is adsorbed to the rotating roller by an adsorption force of toner, sometimes the recording sheet is torn in removing the recording sheet and a part of the torn recording sheet remains adsorbed to the rotating roller. In this state of things, when the image forming apparatus is restarted, the temperature of the rotating roller rises abnormally, which sometimes injures or breaks the rotating roller. When the torn recording sheet remains in the fixing device, there is a risk of breaking a component located in each position of the fixing device or generating positional shift of the component.